The Projects within this Center study the interplay between Ectromelia virus (ECTV), the causative agent of mousepox (the smallpox of the mouse), and its natural host, the mouse. The interplay between ECTV and the immune system of the host can take place at many different levels. It can occur at a tissue level, in which release of immune mediators such as type I interferons alter infection in the skin (Project 1) or lymph node, spleen and liver (Project 2) by ECTV., It can also occur at the cellular level, in which immune cell infiltration and interaction with ECTV infected cells will alter virus replication and spread (Project 1, 3). Finally, it can occur at an intracellular level, in which the trafficking of internalized ECTV, and the endocytic routes used during ECTV infection, can alter the mechanisms by which the immune system senses infection (Project 3 + Aim 2 of this Core). This Core (C) will allow the visualization of all of these events, by providing timely, cost-effective and expert histology and microscopy support for all three Projects within the Center. The equipment and expertise that will be provided by this Core are aimed at reducing the startup time and degree of expertise that would be necessary for individual users if they were required to design and execute experiments and image processing. In addition, the Core will allow imaging of live virus in Biosafety Level 2 containment, which is not possible in shared facilities. In addition to providing support for all three Projects, Aim 2 of this Core will investigate an area of research that bridges areas of investigation in Projects 1 and 3, namely the endocytic processes involved in ECTV internalization and infection, and the presentation of virus-derived components in complex with MHC Class II. Thus, the Core will enhance both the scientific capabilities of the Center and will foster interactions between the individual Projects.